All fossil fuels contain carbon, and all were formed as a result of geologic processes acting on the remains of organic matter produced by photosynthesis—the process by which green plants and certain other organisms transform light energy into chemical energy. Most of the fossil fuel material we use today comes from algae, bacteria, and plants—some of which date back even before the Devonian Period, 419.2 million to 358.9 million years ago. Consequently, at least most of the time, you are not pouring refined dinosaur parts into the gas tank of your vehicle.

Although these carbon compounds are very old, they are not fossils. Although fossils can be the actual remains and traces of ancient plants and animals, they also might be mere impressions made in the rock. With respect to fossil remains, usually only the hard parts of animals, the solid and decay-resistant skeletons, of these organisms are preserved. Shells are made up of calcium carbonate, bones are made up of calcium phosphate, and the glass cases of diatoms are made up of silicon dioxide (silica). If these parts are buried quickly after the death of their owners, surrounding organic tissues might be preserved—yet these soft tissues and hard parts also could become petrified (that is, converted to a stony substance) over time. In addition, sometimes bones, shells, and tissues are transformed into solid minerals; that is, their organic tissues have been completely broken down and replaced by inorganic (or non-living, carbon-free) compounds. Such hardened fire-resistant materials do not make for good energy sources.